Various devices of this type are known, in particular those described in U.S. Pat. No. 3,018,928 (P. Meshberg, Jan. 30, 1962) and in French Pat. Nos. 1 416 685 and 1 503 684 (Idees, Oct. 29, 1965 and Oct. 10, 1966) and 2 300 950 (Valois, Feb. 12, 1975). These devices suffer from the drawbacks firstly of being rather complicated, and consequently expensive and unreliable, and secondly of being rather bulky and thus difficult to integrate into containers having a small opening.